The present invention relates to an idle control device for controlling the idling speed of an engine.
In order to control operating conditions of an engine such as ignition timing, fuel injection and the like, signals are generally utilized which are generated by a signal generator in synchronism with the rotation of the engine. The signal generator generally senses the rotation of a crankshaft or camshaft operatively coupled therewith. An example of this type of signal generator is schematically illustrated in FIGS. 1 and 2. In FIG. 1, a signal generator in the form of a rotational position sensor is generally designated by reference numeral 8 and includes a rotating shaft 1 which is rotated in synchronism with the rotation of a multicylinder engine (not shown) which is, in this example, a four-cylinder engine, and rotary disk 2 secured at its center to the rotating shaft 1 for integral rotation therewith. The rotary disk 2 has a plurality of windows or slits 3 formed therein around the rotating shaft 1 in a circumferentially spaced relation with respect to each other. Each of the slits 3 corresponds to one of the cylinders of the engine, so for a four-cylinder engine, there are four slits in the disk 2. The slits 3 are equally distant from the center of the rotary disk 2. All the slits 3 have the same length as one another in the circumferential direction of the disk 2. Each of the slits 3 has a leading edge L and a trailing edge T. The leading edges L and the trailing edges T of all four slits 3 are equally spaced around the disk 2 at intervals of 90 degrees.
As shown in FIGS. 1 and 2, a light source in the form of a phototransistor 5 are disposed in alignment with each other on opposite sides of the rotary disk 2 in such a manner that when one of the slits 3 is aligned with the light emitting diodes 4 and the phototransistor 5, light emitted from the light emitting diode 4 can pass through the slit 3 thus aligned and reach the phototransistor 5, which is thereby turned on. At other times, the phototransistor 5 remains off.
In operation, when the light which is generated by the light emitting diode 4 passes through one of the slits 3 in the disk 2 and strikes the phototransistor 5, the phototransistor 5 conducts and current flows through the phototransistor 5 and a resistor 5A which is connected to the emitter of the phototransistor 5. An amplifier 6 amplifies the voltage across the resistor 5A and provides the amplified signal to the base of an open-collector output transistor 7.
FIG. 3 illustrates the output signal of the signal generator 8. The output signal is in the form of pulses having a rising edge corresponding to the leading edge L, and a falling edge corresponding to the trailing edge T of each slit 3 in the disk 2. In FIG. 3, a rising edge of an output pulse occurs when the position of the corresponding cylinder is at 5 degrees before top dead center (BTDC), whereas the falling edge occurs when the position of the corresponding cylinder is at 5 degrees BTDC. However, the piston positions corresponding to the rising and falling edges in FIG. 4 are just examples, and different values can be employed.
As shown in FIG. 4, the output signal of the signal generator 8 is inputted to a microcomputer 10 via an interface 9. Based on the output signal from the signal generator 8, the microcomputer 10 controls the ignition timing, the fuel injection, and other aspects of engine operation. For example, in order to stabilize the number of revolutions per minute of the engine, the microcomputer 10 determines successively the instantaneous number of revolutions per minute (rpm) of the engine, for example, by measuring the length of time between the rising or falling edges of two successive pulses of the generator output signal, calculates an average value of the thus determined instantaneous numbers of rpm per a predetermined number of rpm with the corresponding average value so as to obtain a deviation therefrom, and then controls to make a certain appropriate adjustment or modification of the ignition timing in dependence upon the deviation thus obtained.
With the known idle control device as constructed above, however, the target number of rpm during idling is set at a prescribed value irrespective of engine operating conditions, and the number of engine rpm is generally controlled to be maintained at the target value even during a fast idling operation of the engine in which the engine temperature (i.e., engine coolant temperature) is low and the rotational speed of the engine should be held higher than that during the time when the engine temperature is high, so as to enable a more complete combustion of an air/fuel mixture for preventing the degradation of the engine exhaust due, for example, to the increased generation of carbon monoxide, as well as for avoiding the lengthening of an engine warming-up period. As a result, the aforementioned known idle control device is not feasible for controlling the idling operation of the engine over wide operating conditions thereof.
In addition, with the above described idle control device, an idling operation of the engine is detected from the output signals of various sensors such as an idle switch, a boost sensor, a throttle sensor and the like, and when it is detected that the engine is idling, the idle control device performs idle control, i.e., it operates to control the number of engine rpm to be at the prescribed target value. In this case, however, if one or more of the switch or sensors for determining an idling operation of the engine fails, it becomes impossible to determine whether the engine is idling or not so that if there arises an abnormality in the engine operation such as, for example, a great change in the number of rpm, engine stall and the like, the idle control device is no longer able to control the engine operation in an appropriate manner.